rust: sync: atomic: Add Atomic<{usize,isize}>

Add generic atomic support for `usize` and `isize`. Note that instead of
mapping directly to `atomic_long_t`, the represention type
(`AtomicType::Repr`) is selected based on CONFIG_64BIT. This reduces
the necessity of creating `atomic_long_*` helpers, which could save
the binary size of kernel if inline helpers are not available. To do so,
an internal type `isize_atomic_repr` is defined, it's `i32` in 32bit
kernel and `i64` in 64bit kernel.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Elle Rhumsaa <elle@weathered-steel.dev>
Link: https://lore.kernel.org/all/20250719030827.61357-9-boqun.feng@gmail.com/

Author: fbq

rust/kernel/sync/atomic/predefine.rs

@@ -2,6 +2,9 @@
 
 //! Pre-defined atomic types
 
+use crate::static_assert;
+use core::mem::{align_of, size_of};
+
 // SAFETY: `i32` has the same size and alignment with itself, and is round-trip transmutable to
 // itself.
 unsafe impl super::AtomicType for i32 {
@@ -28,6 +31,35 @@ unsafe impl super::AtomicAdd<i64> for i64 {
     }
 }
 
+// Defines an internal type that always maps to the integer type which has the same size alignment
+// as `isize` and `usize`, and `isize` and `usize` are always bi-directional transmutable to
+// `isize_atomic_repr`, which also always implements `AtomicImpl`.
+#[allow(non_camel_case_types)]
+#[cfg(not(CONFIG_64BIT))]
+type isize_atomic_repr = i32;
+#[allow(non_camel_case_types)]
+#[cfg(CONFIG_64BIT)]
+type isize_atomic_repr = i64;
+
+// Ensure size and alignment requirements are checked.
+static_assert!(size_of::<isize>() == size_of::<isize_atomic_repr>());
+static_assert!(align_of::<isize>() == align_of::<isize_atomic_repr>());
+static_assert!(size_of::<usize>() == size_of::<isize_atomic_repr>());
+static_assert!(align_of::<usize>() == align_of::<isize_atomic_repr>());
+
+// SAFETY: `isize` has the same size and alignment with `isize_atomic_repr`, and is round-trip
+// transmutable to `isize_atomic_repr`.
+unsafe impl super::AtomicType for isize {
+    type Repr = isize_atomic_repr;
+}
+
+// SAFETY: The wrapping add result of two `isize_atomic_repr`s is a valid `usize`.
+unsafe impl super::AtomicAdd<isize> for isize {
+    fn rhs_into_delta(rhs: isize) -> isize_atomic_repr {
+        rhs as isize_atomic_repr
+    }
+}
+
 // SAFETY: `u32` and `i32` has the same size and alignment, and `u32` is round-trip transmutable to
 // `i32`.
 unsafe impl super::AtomicType for u32 {
@@ -54,6 +86,19 @@ unsafe impl super::AtomicAdd<u64> for u64 {
     }
 }
 
+// SAFETY: `usize` has the same size and alignment with `isize_atomic_repr`, and is round-trip
+// transmutable to `isize_atomic_repr`.
+unsafe impl super::AtomicType for usize {
+    type Repr = isize_atomic_repr;
+}
+
+// SAFETY: The wrapping add result of two `isize_atomic_repr`s is a valid `usize`.
+unsafe impl super::AtomicAdd<usize> for usize {
+    fn rhs_into_delta(rhs: usize) -> isize_atomic_repr {
+        rhs as isize_atomic_repr
+    }
+}
+
 use crate::macros::kunit_tests;
 
 #[kunit_tests(rust_atomics)]
@@ -73,7 +118,7 @@ mod tests {
 
     #[test]
     fn atomic_basic_tests() {
-        for_each_type!(42 in [i32, i64, u32, u64] |v| {
+        for_each_type!(42 in [i32, i64, u32, u64, isize, usize] |v| {
             let x = Atomic::new(v);
 
             assert_eq!(v, x.load(Relaxed));
@@ -82,7 +127,7 @@ mod tests {
 
     #[test]
     fn atomic_xchg_tests() {
-        for_each_type!(42 in [i32, i64, u32, u64] |v| {
+        for_each_type!(42 in [i32, i64, u32, u64, isize, usize] |v| {
             let x = Atomic::new(v);
 
             let old = v;
@@ -95,7 +140,7 @@ mod tests {
 
     #[test]
     fn atomic_cmpxchg_tests() {
-        for_each_type!(42 in [i32, i64, u32, u64] |v| {
+        for_each_type!(42 in [i32, i64, u32, u64, isize, usize] |v| {
             let x = Atomic::new(v);
 
             let old = v;
@@ -110,7 +155,7 @@ mod tests {
 
     #[test]
     fn atomic_arithmetic_tests() {
-        for_each_type!(42 in [i32, i64, u32, u64] |v| {
+        for_each_type!(42 in [i32, i64, u32, u64, isize, usize] |v| {
             let x = Atomic::new(v);
 
             assert_eq!(v, x.fetch_add(12, Full));